bcachefs: Don't reuse reflink btree keyspace

We've been seeing difficult to debug "missing indirect extent" bugs,
that fsck doesn't seem to find.

One possibility is that there was a missing indirect extent, but then a
new indirect extent was created at the location of the previous indirect
extent.

This patch eliminates that possibility by always creating new indirect
extents right after the last one, at the end of the reflink btree.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -963,7 +963,6 @@ struct bch_fs {
 	struct bio_set		ec_bioset;
 
 	/* REFLINK */
-	u64			reflink_hint;
 	reflink_gc_table	reflink_gc_table;
 	size_t			reflink_gc_nr;
 

fs/bcachefs/reflink.c

@@ -167,24 +167,13 @@ static int bch2_make_extent_indirect(struct btree_trans *trans,
 	if (orig->k.type == KEY_TYPE_inline_data)
 		bch2_check_set_feature(c, BCH_FEATURE_reflink_inline_data);
 
-	for_each_btree_key_norestart(trans, reflink_iter, BTREE_ID_reflink,
-			   POS(0, c->reflink_hint),
-			   BTREE_ITER_SLOTS, k, ret) {
-		if (reflink_iter.pos.inode) {
-			bch2_btree_iter_set_pos(&reflink_iter, POS_MIN);
-			continue;
-		}
-
-		if (bkey_deleted(k.k) && orig->k.size <= k.k->size)
-			break;
-	}
-
+	bch2_trans_iter_init(trans, &reflink_iter, BTREE_ID_reflink, POS_MAX,
+			     BTREE_ITER_INTENT);
+	k = bch2_btree_iter_peek_prev(&reflink_iter);
+	ret = bkey_err(k);
 	if (ret)
 		goto err;
 
-	/* rewind iter to start of hole, if necessary: */
-	bch2_btree_iter_set_pos_to_extent_start(&reflink_iter);
-
 	r_v = bch2_trans_kmalloc(trans, sizeof(__le64) + bkey_bytes(&orig->k));
 	ret = PTR_ERR_OR_ZERO(r_v);
 	if (ret)
@@ -226,7 +215,6 @@ static int bch2_make_extent_indirect(struct btree_trans *trans,
 	ret = bch2_trans_update(trans, extent_iter, &r_p->k_i,
 				BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
 err:
-	c->reflink_hint = reflink_iter.pos.offset;
 	bch2_trans_iter_exit(trans, &reflink_iter);
 
 	return ret;